A typical energy storage and/or generation system can include a direct current (DC) energy storage and/or generation system, a DC bus, a DC-to-alternating current (AC) power conversion system, and an isolation transformer. The DC energy storage and/or generation system can include a battery system containing a plurality of battery cells, and the DC-to-AC power conversion system can include a DC-to-AC bidirectional inverter. The DC bus is disposed between the battery system and the DC-to-AC bidirectional inverter, and the isolation transformer is disposed between the DC-to-AC bidirectional inverter and an AC power grid. The plurality of battery cells can be interconnected within the battery system in series and/or in parallel. For example, the plurality of battery cells can include rechargeable battery cells such as nickel-cadmium battery cells, nickel-metal-hydride battery cells, Lithium-ion battery cells, etc. In a typical mode of operation, the plurality of battery cells are connected to the DC bus, and operate to supply DC electric power onto the DC bus. The DC-to-AC bidirectional inverter converts the DC electric power supplied by the plurality of battery cells into AC electric power, which, in turn, is supplied through the isolation transformer to the AC power grid.
In the typical energy storage and/or generation system described herein, the DC-to-AC bidirectional inverter typically employs a high frequency (e.g., 5 kHz or higher) waveform synthesizer, requiring the AC side of the DC-to-AC power conversion system to be isolated from ground potential. Failure to isolate the AC side of the DC-to-AC power conversion system from ground potential can cause a high frequency AC signal (e.g., 5 kHz or higher) to be impressed on the DC side of the DC-to-AC bidirectional inverter with respect to ground potential, possibly damaging electrical components connected between the DC bus and ground, and/or coupling into noise-sensitive monitoring, control, and/or communication circuits. Such a failure in maintaining proper isolation of the AC side of the DC-to-AC power conversion system from ground potential can result from a ground fault caused by a low resistance or low impedance path from the AC side of the DC-to-AC power conversion system to ground.
Moreover, safety concerns dictate that the DC side of the DC-to-AC power conversion system also be isolated from ground potential. If one side (positive or negative) of the DC bus were inadvertently or deliberately connected to ground potential, then a ground fault occurring on the other side (negative or positive) of the DC bus might result in a dangerously high current condition. In this case, such a ground fault can be caused by a low resistance path from the positive or negative side of the DC bus to ground, or a low resistance path occurring between the series and/or parallel-connected battery cells and ground.
It would therefore be desirable to have more reliable systems and methods of detecting ground faults in energy storage and/or generation systems that employ DC-to-AC power conversion systems that can detect ground faults such as low resistance paths to ground from the DC side of a DC-to-AC power conversion system, as well as low resistance or low impedance paths to ground from the AC side of the DC-to-AC power conversion system. It would also be desirable to have such systems and methods of detecting ground faults in energy storage and/or generation systems that can more reliably generate a warning signal and/or shutdown at least part of an energy storage and/or generation system upon detection of such ground faults within the energy storage and/or generation system.